This application claims priority, under 35 U.S.C. 119, of earlier-filed Japanese application Hei 1-177957, filed July 12, 1989.
The present invention relates to a full hydraulic power steering apparatus for a vehicle, especially an off-highway vehicle such as a tractor or construction vehicle.
The kind of conventional apparatus to be described with reference to an embodiment of the present invention is constructed as follows. The conventional apparatus is provided with a steering unit having a directional control valve operated by a handle, such as the vehicle steering wheel. A steering cylinder is connected with the steering unit through first hydraulic pressure lines comprising a pair of left and right lines, and steers a steered wheel. A pump of a hydraulic power source supplies hydraulic pressure to the steering unit through a second hydraulic pressure line.
In such full hydraulic power steering apparatus, because the handle and steered wheels are not mechanically, but hydraulically, connected, and in spite of the fact that the handle is put in a neutral position, the steering cylinder is frequently displaced from the neutral position due to leakage of hydraulic fluid at the steering unit and steering cylinder, whereby the vehicle may travel zigzag. The applicant of the present invention has proposed the system shown in the Japanese Patent Application No. Sho 63- 321,430, in which a power steering apparatus is disclosed which solves the problem created in the aforesaid power steering apparatus and which can easily amend the actual position of the cylinder with respect to the target position, as set by the position of the handle, without the operator shifting the steering wheel position.
Such power steering apparatus can amend the displacement state with ease without shifting the target position of the cylinder with respect to the handle position. However, when manual steering operation is intended to be carried out during the non-operation of a pump, the problem is created in that there is a fear that discharged oil is not supplied to the steering cylinder, but reversely flows.